Generator Current Imbalance 1

[SomeYahoo]

Hello from a newbie!

I have a question for all of you experts out there. I have a generator application where the customer is specifying both the genset and the equipment. I believe that they have a recipe for disaster with their three phase generator and high, inconsistant single phase loads. The generator is a 10 kW unit and the largest single phase (120V) load is 15 A (20% duty cycle).

Will this imbalance alone cause generator problems? How about if it is in combination with a 28 A/leg (48.4 total amp) 208V three phase load?

I am an ME learning this electrical stuff as I go along. Thanks in advance for all the help!

 

[dpc]

Assuming this is a synchronous generator, unbalanced loads are generally not a major concern.

HOWEVER, you're three-phase load of 28A at 208V is equal to 10 kVA which is close to the unit rating. Assuming the 10kW rating is at 0.8 pf, the **generator** can produce up to 12.5 kVA. What the engine can produce in kW is another story.

The 15A 120V single-phase load adds another 1.8 kVA to one phase, so you would probably be slightly overloading the unit at that point.

If you're unclear on kW versus kVA and three-phase vs single-phase, I'd suggest enlisting help from an EE.

 

[SomeYahoo]

Thanks for the reply dpc.

I understand the basics of power generation, but what I was unsure of was the magnitude of the magnetic effect of the unbalanced load. Having one of the "electromagnets" creating a larger field than the others would cause a mechanical imbalance to be reacted by the generator shaft and bearings. I am concerned that this imbalance could cause vibration and the potential for, um, "mechanical interference" (read: brakage) between the rotor and stator.

Am I out of line with my concern?

 

[dpc]

In a synchronous generator, the field is created by dc current in the rotor. The rotor flux doesn't change all that much from no-load to full-load. But the uneven armature reaction flux will produce some torque pulsation at twice the operating frequency.

If this was a 3600 rpm, 1000 MW machine, significant imbalance might be of some concern. But these small 10 kW engine-generators should not have any problems with single-phase loads, as long as the winding ratings are not exceeded.

It's always best to balance out the single-phase loads among the three phases, to the extent possible.

The generator manufacturer can be consulted if you still have concerns.

 

[edison123]

gensetguy,

At 10 kw and 0.8 pf, the rated current per leg is 35 A. To avoid rotor overheating due to negative sequence flux, it advisable to keep the load current variation in the phases to within 10%.

 

[alehman]

You may wish to check with the manufacturer. Some specify limits on phase imbalance.

 

[SomeYahoo]

Thanks for all the help (red star for dpc for his second post).

I do have a question on Edison's post. Can you please explain "negative sequence flux" and its effects?

 

[edison123]

Negative sequence flux rotates against the positive sequence flux (which is the normal working flux). Since the rotor rotates in the same direction as the positive sequence flux and negative sequence flux rotates in the opposite direction, the rotor sees a flux at double the frequency (ie 120 Hz) which induces its own volatges and current in the rotor winding, heating it up in the process.

Refer any good book on "symmetrical components".

 

[JJHorak]

If you load one phase more than the other, you might get low voltage on that phase. The excitation system only can control one field; it cannot control on a per phase basis. I would throw the Q back at a gen mfr. I would be sort of glad to see what they say is the maximum unbalance loading that should be allowed.